Ethanol made by anaerobic fermentation of sugars by yeast is the major fuel product made from renewable resources in the United States and Brazil. In Brazil, the primary sugar is sucrose, which is readily extracted from sugar cane, one of the most abundant renewable crops grown in that country. In the United States the most abundant renewable crop is corn. The sugar used for the production of ethanol from corn is derived by hydrolysis of the starch contained in the corn endosperm to dextrose (i.e., glucose) which is obtained in a two-step process often called liquefaction and saccharification, whereby the corn starch gelatinized by heating followed by treatment with starch degrading enzymes, in particular α-amylase and glucoamylase to yield glucose monomers and some maltose, with only trace amounts of higher molecular weight saccharides.
Broadly speaking, there are two types of corn processing operations employed in the United States to produce ethanol, commonly referred to as—wet milling and dry milling, the latter often merely called a “dry grind” process. In a classical wet milling operation the starch is highly purified relative to a dry grind process. In wet milling, the corn grain is “steeped” (typically for 22-50 hours at about 50° C.) in an aqueous solution often including small amounts of a mild sulfur acid compound such as sulfur dioxide, sulfuric acid or calcium sulfate which loosens the pericarp (bran) tissue from the interior endosperm and germ tissue. Following the grinding of the steeped corn, the endosperm and germ tissue are separated from each other and from the bran by various filtration and differential density separation techniques. The starch fraction contained in the endosperm is highly purified and therefore easily liquefied and saccharified into dextrose with a relatively low cost for enzyme usage. Ethanol production from an efficient wet milling operation is estimated to be about 2.65 gallons per bushel of corn, however, wet milling to separate corn into its constituent components requires a high cost of equipment and resources, which must be offset by production of higher value products than ethanol. Typically, corn oil is also extracted from the separated germ and a protein enriched product called corn gluten feed or corn gluten meal is extracted from the separated bran, resulting in yet another byproduct denoted “corn fiber” typically used in animal feed.
A conventional dry grind process is much simpler but produces fewer products. In a dry grind operation the grain is subject to grinding to form a coarse whole corn flour that contains exposed starch granules released from the endosperm. The liquification process involves heating the whole ground mixture, which first gelatinizes then ultimately ruptures the starch granules making the starch polymer accessible to acid and enzymatic hydrolysis. The entire crude liquefied mixture is added to form the fermentation media for ethanol production. In the most efficient dry grind processes, approximately 2.75 gallons of ethanol can be obtained per bushel of corn. Because of the relative simplicity of the process, dry grind ethanol production is much less costly from an equipment and resource utilization perspective than a wet milling process although ethanol is the only substantial product made.
Dry grind ethanol production is the most cost effective and efficient process for making ethanol from corn and prior to the present invention, was thought to have reached the maximum output of ethanol possible from crudely ground corn flour. The present invention surprisingly provides still further improvements on the efficiency of ethanol production from a dry grind process without incurring substantial costs or investments in new equipment.